Regional anesthesia involves the introduction of local anesthetics, with the intention of blocking the nerve supply to a specific part of the body so the patient cannot feel pain in that area when a surgical operation is performed, for pain relief during onset of labor or during labor, or for chronic pain. Regional anesthesia is used in both epidural and subdural or spinal procedures, and can involve plexus blocks and blocks of peripheral nerves.
Epidural anesthesia, one form of regional anesthesia, has gained popularity over the years as being an effective manner of blocking pain without requiring entry to the dura mater of the spinal cord (i.e., a spinal anesthesia). In fact, epidural anesthesia is often the anesthesia of choice in child birth. The surgical procedure for epidural anesthesia typically starts with the utilization of a 17- or 18-gauge Touhy needle in the lumbar region in order to puncture the skin, and to traverse at least the supraspinous ligament. The Touhy needle is basically a hollow needle having an angled distal tip which is slightly curved (i.e., a Huber point) and a proximal luer fitting, and a solid stylet which sits inside and substantially fills the hollow needle. Once the skin and supraspinous ligament have been traversed by the Touhy needle, the solid stylet is removed from within the hollow needle, and an air filled syringe is coupled to the proximal luer fitting of the hollow needle. With pressure being applied to the plunger of the syringe as well as to the barrel of the syringe, the hollow needle of the Touhy needle is slowly advanced past the interspinous ligament and ligamentum flavum until the needle enters the epidural space between the ligamentum flavum and the dura mater of the spine. Location of the epidural space which is filled with connective tissue, fatty tissue, and blood vessels is indicated by loss of resistance; i.e., less resistance to the injection of air through the needle. In other words, when the pressure applied by the practitioner to the plunger causes the plunger to readily push air through the needle, the practitioner can assume that the epidural space has been reached. Upon entry to the epidural space, the syringe is carefully disconnected from the hollow needle (extreme care being taken to keep the needle in its exact position), and a catheter is threaded through the hollow needle. Because the hollow needle has an angled distal end, upon reaching the distal end of the hollow needle, the catheter is directed into the epidural space which is substantially perpendicular to the direction of the needle. The catheter is advanced only two to three centimeters into the epidural space in order to reduce the likelihood that it might exit though an intervertebral foramen, with resulting inadequate epidural anesthesia. With the catheter in place, a test dose, repeated injections, or a continuous flow of anesthesia may be administered through the catheter.
A conventional method of placing regional anesthetic needles is to use anatomical landmarks, tapping on the barrel of an attached syringe to feel the way forward until a loss of resistance is obtained. Because the person placing the needle must rely on tactile information, the incidence of errors such as dural punctures, wet taps during lumbar epidurals, and spinal chord injury during cervical or thoracic epidural placements is a serious problem.
While the apparatus and methods for administering regional anesthesia have proved successful over a long period of time, there are drawbacks to those approaches and methods. With regard to the Touhy needle in particular, although it is provided with a curved Huber point, the Touhy needle is still sharp. Use of the Touhy needle therefore runs the risk that the practitioner, i.e., physician, operator, or other person trained to perform this procedure, might overshoot the epidural space and enter the subdural space between the dura mater and the arachnoid mater of the spine, or the subarachnoid space. Such a mistake could result in extreme over-application of anesthesia with a possible high level of spinal anesthesia, necessitating endotracheal intubation of the trachea and mechanical ventilation of the patient. Similar complications could also occur during the disconnection of the syringe from the Touhy needle and insertion of the catheter, as the patient might move, or the needle might not be held properly in position. Such movement of the needle could result in the undesirable entry of the needle and/or catheter into the subdural or subarachnoid spaces.
In order to obtain better information during regional anesthesia needle placement, it has been proposed to use X-ray fluoroscopic techniques to place the needles, but in many cases it is not possible to use X-ray, for example in the case of pregnant women or at the bedside in the trauma intensive care unit (TICU).
Recently it has been proposed to use 2-D echo to facilitate placement of regional anesthetic needles. MicroMaxx™ hand-held ultrasonic devices, manufactured by SonoSite, Inc., which use 2D ultrasound on the patient's skin, have been used at Dartmouth-Hitchcock Medical Center to assist in proper placement of the needle. The use of a 2D echo device is complicated by the need for the use of an acoustic gel, which can compromise sterility when used to assist with regional anesthetic needle placement. Moreover, the 2D echo device is not useful for imaging structures inside the bony spinal canal because the structure of bone tissue scatters ultrasound randomly, such that no intelligible image can be obtained from sound reflected from tissue structures that are deep to bone.
Accordingly there exists a long-felt and continuing need for apparatus and methods for placement of epidural needles and other regional anesthesia needles.